Predicting Emissions Using CFD Simulations of an E30 Gasoline Surrogate in an HCCI Engine with Detailed Chemical Kinetics 2010-01-0362

To accurately predict emissions as well as combustion phasing in a homogeneous charge compression ignition (HCCI) engine, detailed chemistry needs to be used in Computational Fluid Dynamics (CFD) modeling. In this work, CFD simulations of an Oak Ridge National Laboratory (ORNL) gasoline HCCI engine have been performed with full coupling to detailed chemistry. Engine experiments using an E30 gasoline surrogate blend were performed at ORNL, which included measurements of several trace species in the exhaust gas. CFD modeling using a detailed mechanism for the same fuel composition used in the experiments was also performed. Comparisons between data and model are made over a range of intake temperatures. The (experiment & model) surrogate blend consists of 33 wt % ethanol, 8.7 % n-heptane and 58.3 % iso-octane. The data and simulations involve timing sweeps using intake temperature to control combustion phasing at a constant fuel rate. The modeling uses a detailed chemical kinetic mechanism consisting of 428 species and 2378 reactions. This mechanism was obtained by a targeted mechanism reduction of a well validated master kinetics mechanism for multiple gasoline surrogate-fuel components, which consists of 3553 species and 14904 reactions. A 15-degree sector mesh consisting of 53,800 cells at IVC has been used for the closed valve simulations.

The CFD simulation employs the newly developed FORTÉ simulation package, which was designed to take advantage of advanced chemistry solver methodologies as well as advanced spray models. In this study, there is no spray model used, since the fuel is atomized and quickly vaporized during port injection. However, parallel computing, dynamic adaptive chemistry and dynamic cell clustering methods have been used to minimize the chemistry related computational time while maintaining accuracy in the kinetics predictions. These methods allow inclusion of the relatively large (428 species) detailed kinetics mechanism directly in the simulation, while keeping the overall simulation time reasonable for production work.

Comparisons with the engine data include the trends of combustion phasing as a function of intake temperatures. Emissions of several species are also compared with engine data. The ORNL engine experiments included detailed exhaust measurements of NO_x, CO, formaldehyde, acetaldehyde, methane, ethylene, propene, iso-butylene and the overall unburned hydrocarbons. All of these exhaust measurements have been compared with the modeling results, as a function of intake temperatures. The results agree well with the engine data, and the agreement provides confidence in the predictive capability of the model for studying chemistry and fuel effects.